One possible treatment involves fusing the three bones represented by the calcaneus, astragalus and tibia, as is proposed in US-A-2005/0107791. A long compression screw internally connects the tibia and the calcaneus by passing through the astragalus, in such a way as to block these three bones relative to one another. This results in a considerable loss of mobility for the patient, since the ankle is totally blocked.
Another possible treatment involves fusing the calcaneus and astragalus, with the aid of a dedicated arthrodesis rod and re-establishing the articular capacity between the astragalus and tibia, by implanting an ankle joint prosthesis. However, the two articulations then have to be treated in two stages: the arthrodesis of the calcaneus and astragalus requires a consolidation period of at least six months, which delays the implantation of the articular prosthesis between the astragalus and the tibia.
US-A-2005/0288792 discloses an astragalar component of an ankle joint prosthesis with which it is possible to re-establish the articular capacity between the astragalus and the tibia, while at the same time using an astragalocalcanean rod that extends from a lower side of the astragalar component to the upper part of the calcaneus, in so doing passing right through the astragalus. In one embodiment, the upper end of this rod is screwed directly into internal threads of the astragalar component, which threads open out on the lower side of this component. Before being implanted, the component and the rod are firmly immobilized relative to each other by screwing. When this screwed assembly is implanted, the rod guarantees good anchoring of the astragalar component, since the latter is thus fixed both to the astragalus and also to the upper part of the calcaneus, which explains why the prosthetic assembly is more specifically intended for revision of an arthroplasty of the ankle. However, the assembly does not effectively block the articulation between the calcaneus and the astragalus, since relative movements around and/or along the rod remain possible and, in practice, appear quickly.
FR-A-2 220 235 proposes a prosthetic ankle assembly comprising an astragalar component of an ankle joint prosthesis in the form of a support designed to be fixed to the astragalus of a patient. This support is intended to cooperate in an articulated manner with a tibial rod, with interposition of an intermediate articulated component. In its lower part, the astragalar support is joined to three anchoring feet, namely two anterior feet and one posterior foot, which are intended respectively to be inserted into corresponding cavities formed in the astragalus and in the calcaneus. As regards the posterior foot, one embodiment entails this foot being formed by at least one sagittal screw that passes through the support from top to bottom, until bearing with its head on the support, while its stem is inserted, without being screwed, into the cavity of the calcaneus, plugged with cement. The benefit of using such a screw is that, if necessary, it can be unscrewed in order to facilitate the removal of the support. Thus, the posterior foot of this prosthetic assembly provides a stable means of bearing on the calcaneus, but, even in the form of a screw inserted into cement, this foot does not totally block the articulation between the astragalus and the calcaneus in an effective manner.
WO-A-01/30264 and US-A-2005/124995 for their part disclose a prosthetic ankle assembly comprising an astragalocalcanean rod that is introduced into a cavity formed beforehand through the astragalus and in the upper part of the calcaneus. This rod is used to anchor an astragalar plate, which is fitted in place and fixed to the astragalocalcanean rod after the rod has been placed in the aforementioned bone cavity. Hence, this rod does not provide any effective blocking of the articulation between the calcaneus and the astragalus so that, in order to limit the relative movements between these two bones, additional screws are fitted between the astragalar plate and the astragalocalcanean rod, in order to stabilize the position of the latter.